This invention relates to a control device for controlling a thyristor operatively coupled to an electric discharge device such as a flash discharge lamp, and more particularly to a control device effective for preventing the permanent breakdown of such a thyristor.
It has been already known to use thyristors to control the energy of flashing light emitted by flash discharge lamps. The thyristor is operative to permit the associated flash discharge lamp to emit a flashing light for a time interval between a time point at which the thyristor is turned on and a time point at which it is turned off so that the energy of flashing light from the flash discharge lamp can be controlled by changing the ON time period of the thyristor. Also there have been previously proposed attempts to automatically control the energy of flashing light from flash discharge lamps to a predetermined magnitude. Such flash discharge lamps are principally used as stroboscopic tubes for use with photographic cameras.
That thyristor controlling such a flash discharge lamp is serially connected to the flash discharge lamp and includes an ignition circuit for bringing it into its ON state and a commutation circuit for resetting it to its OFF state. The commutation circuit has usually a commutation capacitor connected to the thyristor to apply a reverse voltage to the thyristor dependent upon an electrical charge accumulated thereon when the thyristor is to be reset to its OFF state. After having applied to the reverse voltage to the thyristor, the commutation capacitor is charged with the opposite polarity. Thyristors are generally transferred to their OFF state in response to the application of the reverse voltage thereto but a transient state exists until the thyristors are completely returned back to their OFF state. In that transient state the thyristors are apparently in their OFF state but they can readily be turned on. If, in the transient state, a thyristor is again turned on to permit the associated commutation capacitor charged with the opposite polarity to flow a discharge current through the conducting thyristor, then the thyristor may frequently permanently breakdown so that the thyristor is always conducting across the anode and cathode electrodes thereof.